The purpose of this project is the collaborative study of the physical properties of a wide variety of biological macromolecules with the goal of correlating these properties with the structure and function of the macromolecules. Analytical ultracentrifugation and mathematical modeling are the principal research techniques used. Collaborative studies with the laboratory of Dr. Samuel Wilson (NIEHS) on proteins involved in DNA transcription initiation and in DNA repair have been continued. These have included work on the interactions between DNA Ligase I and the replication protein, proliferating cell nuclear antigen (PCNA); the interaction mechanisms between the XRCC1(1-183) protein and DNA polymerase-beta and its subdomains; the interactions between AP endonuclease and DNA, DNA polymerase-beta and DNA, and both together with DNA. Research has been completed on two aspects of the studies on DNA enzymes, the thermodynamics of oligomerization of PCNA and the interaction of PCNA with histidine-tagged and untagged FEN-1. Two manuscripts are in preparation. Collaborative studies on DNA transcription initiation repression by gal repressor (galR) and the HU protein have been conducted with the laboratory of Dr. Sankar Adhya (NCI). It has been demonstrated that a Gal repressor-operator-HU ternary complex can be formed and that this represents a pathway of Gal repressosome assembly which governs the transcription of two tandem galpromotors in E. coli. Studies on the associative behavior of translin, a protein involved in translocation of chromosomal DNA, have been done in a collaboration with Dr. Jay Knutson (NHLBI) and Dr. Myun Ki Han (Pioneer Biotechnology), are essentially complete. New collaborative studies with the laboratory of Dr. Richard Youle (SNB, NINDS) on the mechanism of dimerization of Bcl-X1 have been initiated. Bcl-X1 is a member of the Bcl-2 family of proteins, which are central regulators of apoptosis in promoting or inhibiting cell death. We have found that the presence of sub-critical micelle-forming concentrations of detergents such as Triton X-100 and 1.0 M urea are needed to facilitate dimerization, suggesting that significant conformational changes are required to permit dimerization. We are investigating the solvent conditions that will optimize dimerization and attempt to determine the conformational changes that are involved in this transition. This is critical for understanding the role of this protein in apoptosis, since, in the normal healthy cell, it is found only in the monomeric form; dimer is found only under apoptotic conditions. This work is still at a very early phase, and the most definitive finding to data appears to be that the monomer-dimer reaction does not appear to be reversible or to exhibit significant temperature dependence.